It is well known to use hose for the transport of fluids, such as liquefied gases, at low temperature. Such hose is commonly used to transport liquefied gases such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG).
In order for the hose to be sufficiently flexible, any given length must be at least partially constructed of flexible materials, i.e., non-rigid materials.
The present invention is directed to composite hose. Conventional composite hoses are made of layers of polymeric films and fabrics sandwiched between an inner and outer helical metallic wire. The hose is constructed by wrapping around a mandrel, in sequence, the inner wire, combinations of films and fabric, and the outer wire. The inner and outer wires have the same helical pitch but are offset by half the pitch length to form a corrugated hose wall profile. The resulting tubular structure is then extracted from the mandrel and terminated with end fittings. The end fittings are typically constructed of a metallic tail and a ferrule. The tail has two parallel helical groves machined into the outer surface which matches the double helix formed by the inner and outer wires. The tail is inserted into the bore of the hose with a ferrule on the outside. Depending on the application, the end of the hose pack may be bound, capped with a rubber cuff or impregnated with a two part epoxy resin, and the ferrule is then crimped or swaged down on to the tail to retain the end of the hose. A hose of this general type is described in European patent publication no. 0076540A1. The hose described in this specification includes an intermediate layer of biaxially oriented polypropylene, which is said to improve the ability of the hose to resist the fatigue caused by repeated flexing.
In our earlier patent application WO01/96772, we described a new composite hose which incorporated a braid with the film and fabric layers sandwiched between the two helical wires. We also described a new end fitting for this hose. Further improvements to the hose and end fitting were described in our patent applications WO04/044472 and WO04/079248. These composite hoses may be provided with a large bore and are typically aimed at ship to ship fluid transfer operations which are governed by the requirements of the International Maritime Organisation (IMO). The IMO requirements for hoses (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk—the “IGC Code”) are demanding (for reasons of safety) that the hose burst pressure must be five times the maximum working pressure at the extreme service temperature. The maximum working pressure typically ranges from the minimum required by IMO of 10 barg up to 20 to 30 barg.
The inner and outer wires in composite hoses are conventionally made from steel. When the hose is to be used for cryogenic service, austenitic steels will be used.
Exemplary austenitic stainless steel grades for cryogenic service are the “series 300” which do not exhibit low temperature brittleness. The important material properties are the yield stress (YS), the yield strain (EY), the ultimate tensile strength (UTS), the failure strain (EF), the elastic modulus (E), the density (RHO), the thermal conductivity (K) and the thermal expansion coefficient (CTE). These properties vary over the range from ambient (293° K) to cryogenic temperatures (4° K for Liquid Helium or 77° K for Liquid Nitrogen [LN2]). In general, the strength increases with reducing temperature. This is illustrated by considering as an example AISI grade 304 (8 g/cc density) which is a commonly used austenitic stainless steel for cryogenic service. The YS & UTS of 304 at room temperature is about 250 MPa & 590 MPa respectively, and at LN2 temperature (77° K) about 400 MPa & 1525 MPa respectively. While there is some reduction in the ductility with EF reducing from 60% at ambient temperature to 40% at LN2 temperature, there is more than adequate ductility with 304 at this cryogenic temperature. Although this increase in strength is considered beneficial, designers of cryogenic pressure vessels tend to rely on the minimum ambient temperature specifications. The ambient LN2 temperature elastic moduli for 304 are 193 GPa and 205 GPa respectively.
An important design issue for cryogenic equipment is the effects of the dimensional changes and thermal gradient transients associated with the circa 215° K temperature change from room ambient to cryogenic service conditions. Steels such as 304 are thermally conductive and they will contract with decreasing temperature. The thermal conductivities for 304 at room temperature and LN2 temperature are 8 & 15 W/m.° K respectively. The average CTE over this temperature range is 13×10−6° K−1 i.e. a length contraction of about 3 mm/m for this temperature difference of 216° K.
In order to maintain the hose wall arrangement it is important to maintain the tension in the outer wire. Because the wall pack of the hose is made up of a thick layer of films and fabrics it has inherently good insulation properties and therefore there is a temperature difference between the inner and outer helical wire when in cryogenic service. Therefore the inner wire will contract more than the outer wire and this is compensated for by the residual tensions in the respective wires introduced during manufacture.